1. Field of the Invention
The present invention relates to a magnetic toner with a negative polarity for developing latent electrostatic images in the fields of electrophotography and electrostatic printing, and an image forming apparatus using the above-mentioned magnetic toner. In addition, the present invention also relates to a method for measuring the amount of a charge control agent extracted from the surface of magnetic toner particles by allowing a solvent to selectively dissolve the charge control agent therein.
2. Discussion of Background
To develop latent electrostatic images formed on the surface of a latent electrostatic image bearing member such as an electrophotographic photoconductor or electrostatic printing member, a wet type development method and a dry type development method are currently available. The wet type development employs a liquid developer, while the dry type development employs a one-component developer comprising a toner which comprises a binder resin and a coloring agent dispersed therein, or a two-component developer comprising the above-mentioned toner and a carrier. Although these development methods have their own advantages and disadvantages, the dry type development is particularly widely used.
Among the above-mentioned development methods, the one-component development method is advantageous because the system for controlling the concentration of toner is not necessary, and accordingly, stirring means in the development unit can be simplified. This can reduce the size of the image forming apparatus, so that the one-component development method has been particularly adapted in the field of printer.
The one-component development method is divided into a magnetic development system and a non-magnetic development system. The former system adapts the combination of a magnetic toner and a magnetic development sleeve; the latter adapts the combination of a non-magnetic toner and a non-magnetic development sleeve. More advantages can be found in the magnetic one-component development system than in the non-magnetic system.
Whether the toner is a magnetic toner or non-magnetic toner, a desired charge quantity of toner cannot be obtained merely by dispersing a coloring agent such as a dye or pigment in a binder resin. A charge control agent is usually added in a proper amount to the formulation of toner. However, when the charge control agent is contained in the toner, the problem of a toner film being deposited on the development sleeve tends to easily occur. This problem is hereinafter referred to as toner filming. Although the initial image quality and initial development properties are satisfactory, the performance of the charge control agent is lowered for an extended period of time, if the toner filming occurs, thereby impairing the image quality of the obtained toner images and the development properties. The decrease of image quality and development properties is considered to result from the performance of the charge control agent itself, and the dispersion properties of the charge control agent in the toner particles. In particular, the dispersed condition of the charge control agent in the surface portions of toner particles, which come in direct contact with the development sleeve, largely affects the toner filming problem. It is generally supposed that the occurrence of toner filming is accelerated in the development of an image with a small area because such a toner image stays on the development sleeve for a long period of time.
In the two-component development method, special attention has been paid to the dispersed condition of a charge control agent in the surface portion of toner particles.
For example, Japanese Laid-Open Patent Applications 8-12309, 61-36767, and 5-150554 report that the charging characteristics of toner can be stabilized, and the problems of spent toner, toner scattering, and fogging can be effectively reduced by measuring and controlling the concentration of charge control agent in the surface portions of the toner particles by absorptiometric method or potential difference method.
In the above-mentioned conventional methods of measuring the concentration of charge control agent, the toner particles are placed in a solvent which consists of such an alcohol that cannot dissolve the employed binder resin therein, but can dissolve the charge control agent therein. These methods are evaluated as effective for the non-magnetic toner. However, as to the case of the magnetic toner, there is no description about the specific effect of such measuring methods in the above-mentioned applications.
The inventors of the present invention have tried to measure the concentration of a charge control agent existing in the surface portion of toner particles for use in a magnetic toner in accordance with the above-mentioned conventional methods. As a result, such conventional methods proved to be impractical when used for the magnetic toner. This is because all the charge control agent, not only existing in a surface portion, but also in an inner part of the magnetic toner particles, was dissolved in the solvent with the elapse of time. It became clear that the concentration of the charge control agent in the surface portion of the magnetic toner particles cannot be precisely measured.
In general, a polymeric material, such as polystyrene, serving as a binder resin for use in a toner used for developing latent electrostatic images is known to be insoluble in an alcohol, but apt to swell therein. On the other hand, a magnetic powder for use in the magnetic toner is insoluble in an alcohol, and does not swell therein. Therefore, when such a magnetic toner is placed in an alcohol solvent, gaps are generated between the polymeric material serving as a binder resin and the magnetic powder, and the alcohol penetrates into the inside of the toner particles through the above-mentioned gaps. The result is that even the charge control agent existing in the inner part of the toner particles is dissolved in the alcohol.
It is therefore a first object of the present invention to provide a magnetic toner for developing latent electrostatic images, capable of preventing the toner filming problem and producing high quality toner images for a long period of time.
A second object of the present invention is to provide an image forming apparatus using the above-mentioned magnetic toner.
A third object of the present invention is to provide a method for measuring the amount of a charge control agent extracted from the surface of toner particles of a magnetic toner comprising the charge control agent.
The above-mentioned first object of the present invention can be achieved by a magnetic toner with a negative polarity for developing latent electrostatic images, comprising toner particles, each toner particle comprising a binder resin, a magnetic powder, and a charge control agent, the surface of the magnetic toner satisfying the relationship of 2xc3x9710xe2x88x927xe2x89xa6C/Swxe2x89xa610xc3x9710xe2x88x927, provided Cxe2x89xa68xc3x9710xe2x88x923, wherein C is an amount (g/g) of the charge control agent dissolved in a solvent comprising water and methanol per unit amount (g) of the toner particles, and Sw is a specific surface area (cm2/cm3) determined from a weight-average particle diameter of the toner particles.
The second object of the present invention can be achieved by an image forming apparatus comprising a member for bearing an electrostatic image thereon, and developing means for developing the electrostatic image with a developer, that is, the above-mentioned magnetic toner with a negative polarity, comprising a developer holding member provided with a magnet, and a developer container for storing the developer therein.
The third object of the present invention can be achieved by a method for measuring the amount of a charge control agent extracted from the surface of toner particles of a magnetic toner comprising the charge control agent, using an aqueous solvent comprising water and an alcohol, comprising the steps of wetting the magnetic toner with water, adding an alcohol to the magnetic toner wetted with water, stirring the mixture of the magnetic toner wetted with water and the alcohol added, thereby extracting the charge control agent from the surface of the toner particles with a mixture of the alcohol and water, which constitutes the aqueous alcohol solvent, the aqueous alcohol solvent comprising 15 to 30 vol. % of water, and 70 to 85 vol. % of the alcohol with the total volume percentage of the aqueous alcohol solvent being 100 vol. %, and separating the toner particles from the aqueous alcohol solvent, using a magnet, to obtain an extract liquid in which the charge control agent is dissolved.